Method and apparatus for reduction of fluid borne noise in hydraulic systems

ABSTRACT

Method and apparatus for reduction of fluid borne noise in a hydraulic system are disclosed. An improved isolating hose assembly comprising first and second volumetrically compliant members with an inductive flow member providing fluidic coupling therebetween is disclosed. The improved isolating hose assembly acts similarly to an electronic low pass   filter network in deterring fluid borne noise from entering portions of the hydraulic system located therebeyond. In addition, a sound barrier comprising a heavy-walled boot surrounding the first volumetrically compliant member is disclosed. The heavy-walled boot surrounding the first volumetrically compliant member acts to prevent acoustic sound sourced at the wall surface thereof from entering the surrounding atmosphere.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to hydraulic systems and, moreparticularly, to significantly reducing fluid borne noise commonlypresent in such systems with particular reference to reduction of noisein vehicular power steering systems.

II. Description of the Prior Art

Fluid borne noise is commonly present in hydraulic systems powered bypumping apparatus such as gear, vane or piston pumps. Typically, thenoise results when pressure waves are generated as pump flow rippleencounters system flow impedances. Flow ripple can be generated by thepumping apparatus as a combination of any, or all, of the followingthree types of disturbances:

1. Kinematic flow ripple, which is a function of pump geometry. By wayof example, the flow output of a piston pump is generally a summation ofan odd number of pistons moving in a sinusoidal manner. This results inflow ripple whose fundamental frequency is equal to the product of thepump's rotational speed and the number of pistons.

2. Compression flow ripple, which is a result of compression, ordecompression, of a trapped fluid volume. Compression flow ripple iscommonly encountered in gear pumps as the pump's gears mesh. This isbecause the gears mesh with a contact ratio greater than one wherebyfluid can be trapped between succeeding sets of teeth whenever they areconcomitantly in contact.

3. Leakage flow ripple, which is a result of pressure differentialsacross varying leakage paths formed between moving and stationary pumpcomponents.

Fluid borne noise present in a hydraulic system causes mechanicalapparatus such as hydraulic lines, control valves, hydraulic motors, andsupporting structural members to vibrate. In many cases such vibrationis coupled to the atmosphere and is the source of objectionable acousticnoise. It is desirable to attenuate such fluid borne noise near ahydraulic system's pumping apparatus thereby isolating the rest of thesystem and minimizing vibration and resulting acoustic noise.

Fluid borne noise reduction apparatus of the prior art usually comprisesa flexible metal tube, called a tuning cable, placed inside a section ofvolumetrically compliant hose. Such prior art apparatus is described inU.S. Pat. No. 3,323,305 entitled ATTENUATION DEVICE and issued to G. T.Klees in June 1967. Although such tuning cable designs are based upondestructive interference principles, their performance has never beenfully analyzed. For instance, the automotive industry has resorted toempirical methods and subjective evaluations in applying the technologyto vehicular power steering systems wherein its success has beenmarginal at best. The following quotation from SAE Technical Paper No.931295 entitled ANALYSIS OF TUNING CABLES FOR REDUCTION OF FLUIDBORNENOISE IN AUTOMOTIVE POWER STEERING HYDRAULIC LINES by M. C. Hastings andC. C. Chen given in May 1993 (which paper comprises mathematicaltechniques for analysis of the prior art technology) illustrates thispoint:

"The distributed parameter mathematical model does accurately predictattenuation of the pressure wave in simple (straight line) systems. Inmore complex systems, however, overall attenuation is a function ofconfiguration, including the number of discontinuities and supportpoints, and varies as a function of frequency. In real systems with manycurves, bends and other discontinuities, the complex fluid-structureinteraction dominates and tuning cables may have little, if any, effecton noise reduction in the frequency range of interest."

As a matter of fact, this analysis misses the mark slightly because thesupposed destructive interference usually does not occur. This is forthree reasons. Firstly, the pumping apparatus acts in the manner of aflow source rather than a pressure source in issuing the offending noisesignal. Thus, it presents a substantially infinite source impedance tothe tuning cable. Secondly, because the tuning cable is spiral woundwith a discontinuous wall, it has distributed leakage therethrough.Thirdly, because the expandable nature of the volumetrically complianthose, effective bulk modulus of fluid flowing therewithin is as much as30 times smaller than the fluid itself. Thus, sound velocity andtherefore wavelength are up to about 5.5 times less than within thetuning cable. In effect, prior art noise reduction apparatus dominantlycomprises a distributed coupling of pressure to the volumetricallycompliant hose which substantially acts like a continuous array ofcapacitors each coupled to ground.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to method and devices forsubstantially eliminating fluid borne noise in hydraulic systems, andadditionally for reducing concomitant coupling to the atmosphere in theform of acoustic noise. The methods and devices are particularlydirected to eliminating power steering pump (hereinafter refereed to as"pump") sourced noise from vehicles comprising a power steering system.

In a vehicular power steering system its pump is generally mounted tothe vehicle's engine via a suitable mounting bracket. As such, any pumpsourced mechanical noise is coupled directly to the engine. And, theengine is presumed to be isolated from the vehicle in a manner thatprovides mechanical noise isolation. However, any fluid borne noisereaching the system's steering gear can be coupled to both a mountingsubframe and the steering column wherefrom undesirable acoustic noiseemanates.

In a preferred embodiment, an improved isolating hose assemblyattenuates substantially all discernible fluid borne noise incidentthereupon, thus effectively isolating the remainder of a host hydraulicsystem from its pump sourced fluid borne noise. The improved isolatinghose assembly is quite simple in nature, comprising a firstvolumetrically compliant member followed by a relatively small diameterinductive flow member and a second volumetrically compliant member. Asdescribed below, the improved isolating hose assembly acts similarly toan electronic low pass filter network optimized for use with a currentsource.

In analyzing the improved isolating hose assembly, the pump is regardedas an alternating flow source of fluid borne noise. The firstvolumetrically compliant member functions like a capacitor to ground forpreferentially bypassing alternating fluid flow components of ahydraulic signal. The inductive flow member functions like a seriesinductor for impeding further transmission of remaining alternatingfluid flow components. And the second volumetrically compliant memberfunctions like another capacitor to ground for preferentially bypassingremaining alternating fluid flow components of the hydraulic signal. Aload comprising a control valve and hydraulic motor is coupled to theimproved isolating hose assembly via hydraulic tubing which acts like atransmission line having relatively high characteristic impedance.Generally the load is resistive in nature, and may be of either higheror lower impedance than the hydraulic tubing.

Utilization of the improved isolating hose assembly results insignificantly reduced fluid borne noise level in either the hydraulictubing or the load over frequencies of interest. However, significantacoustic noise may still be present. This is because the firstvolumetrically compliant member is normally formed of volumetricallycompliant hose whereby higher frequency harmonics, should they bepresent, can drive its surface to significant levels of vibration. Suchhigh frequency surface vibration can be sufficient for coupling soundenergy into the surrounding atmosphere in a manner similar to the actionof a loudspeaker.

Therefore, in a first alternative preferred embodiment, the firstvolumetrically compliant member is surrounded by a non-contactinghousing whose characteristic acoustic impedance differs substantiallyfrom that of the atmosphere surrounding the first volumetricallycompliant member. As a result, acoustic noise is reflected inward andthus prevented from entering the surrounding atmosphere. As depictedbelow, the non-contacting housing is formed in the manner of aheavy-walled elastomeric boot surrounding the first volumetricallycompliant member. The elastomeric boot comprises end sections adaptedfor attachment to fittings located at either end of the firstvolumetrically compliant member.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will beapparent from the written description of the drawings, in which:

FIG. 1 is a cross-sectional view of the improved isolating hose assemblyin accordance with the invention;

FIGS. 2A and 2B are equivalent circuit diagrams useful for respectivelycomparing performance of the improved isolating hose assembly of thepresent invention with noise reduction apparatus of the prior art; and

FIGS. 3A and 3B are graphs depicting improvement in fluid borne noisesuppression achieved by utilizing the improved isolating hose assemblyof the present invention in place of noise reduction apparatus of theprior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An improved isolating hose assembly 10 of the present inventioncomprising first and second volumetrically compliant members 12 and 14,respectively, and an inductive flow member 16 is shown in FIG. 1.Generally, first and second conduits or volumetrically compliant members12 and 14, respectively, are formed of high expansion hose such as DaycoNo. DF 3289 (i.e., which has a passage having an inside diameter of0.375 in. and having walls 62 and 63 which expand to increase the volumeof the passage expands by about 50% at 1,300 psi). In addition, improvedisolating hose assembly 10 comprises input and output sections 18 and20, respectively.

Input section 18 includes tubing 22 formed with a taper section 24,"ripple" section 26, and axial position locating flange 28. An internalnut 30 is positioned upon portion 32 of tubing 22, an O-ring flange 34is formed and an O-ring 36 mounted on tubing 22. Then a first collarfitting 38 comprising internal ridges 40 is positioned over "ripple"section 26 and against axial position locating flange 28. The inputsection 18 is inserted in input end 42 of first volumetrically compliantmember 12 and first collar fitting 38 is crimped in a known manner toform a pressure-tight connection between input end 42 and "ripple"section 26.

The inductive flow member 16 is formed in a tubular manner and comprisesan external ridged section 46. It is inserted into a selected locationto form a flow barrier between first and second volumetrically compliantmembers 12 and 14, respectively, whereby flow is conveyed to the secondvolumetrically compliant member 14 via bore 17 formed in inductive flowmember 16. Then collar 48 is positioned over ridged section 46 andcrimped as before to fixedly locate the inductive flow member 16 betweenfirst and second volumetrically compliant members 12 and 14,respectively, and make the resulting flow barrier pressure tight.

Output section 20 is depicted as a single piece formed of output fitting50 and second collar fitting 52 joined as by brazing along line 54. Asshown, output fitting 50 is formed with a female boss 56 adapted forreceiving a delivery tube assembly (not shown) having an input sectioncomprising another internal nut 30, O-ring flange 34 and O-ring 36grouping. In any case, output section 20 is inserted in output end 58 ofsecond volumetrically compliant member 14 and second collar fitting 52is crimped to form a pressure-tight connection between output end 58 andridged section 60 of output fitting 50 as before.

Operationally, improved isolating hose assembly 10 is used within a hosthydraulic system as follows: O-ring flange 34 (with O-ring 36 mountedthereon) is inserted in an output boss associated with a pump (neithershown) and the rest of the host hydraulic system is connected to outputfitting 50 via the delivery tube assembly. In general, the pump issuesfluid flow comprising both constant and alternating flow components tothe improved isolating hose assembly 10. The alternating flow componentflows within the first volumetrically compliant member 12 and ispreferentially bypassed out of the hydraulic system (i.e., in a mannersimilar to a bypass capacitor) by displacing wall 62 thereof in analternating manner. This is due in part to further transmission of thealternating flow component being impeded by inductive flow member 16.Thus, within the improved isolating hose assembly 10, the overwhelmingmajority of the alternating flow component is limited to the firstvolumetrically compliant hose section 12. And, in the event that theimpedance of the rest of the host hydraulic system, as seen from outputfitting 50, is large compared with that of the second volumetricallycompliant member 14, any remaining alternating flow component ispreferentially confined thereto via similarly displacing wall 63.

As mentioned above, fluid borne noise reduction apparatus of the priorart acts substantially like a continuous array of capacitors, eachcoupled to ground while the improved isolating hose assembly 10 of thepresent invention acts substantially like a filter network. Inoperation, the intended purpose of either apparatus is to absorb most ofthe alternating flow component issuing from the pump and deter it fromentering the rest of the system.

As an aid to comparing improved isolating hose assembly 10 of thepresent invention with fluid borne noise reduction apparatus of theprior art it is expedient to utilize equivalent circuits 64 and 66,respectively, as schematically shown in FIGS. 2A and 2B, respectively,for this purpose. In equivalent circuit 64, a current source (i.e.,emulating the alternating flow component issuing from the pump) 68 iscoupled to a transmission line 70 (i.e., emulating the delivery tubeassembly) and a resistive load impedance 94 (i.e., emulating the controlvalve and hydraulic motor) via a filter network 72. In equivalentcircuit 66, the current source 68 is coupled to the transmission line 70and the resistive load impedance 94 via a short length of line 74coupled to capacitor 76, respectively.

In FIG. 2A the filter network 72 comprises a first capacitor 78 (i.e.,emulating the first volumetrically compliant member 12), inductor 80(i.e., emulating the inductive flow member 16) and a second capacitor 82(i.e., emulating the second volumetrically compliant member 14). Inaddition, the capacitors 78 and 82, and inductor 80 are depicted inseries with resistors 84, 86 and 88, respectively, to account for energyloses in each of first and second volumetrically compliant members 12and 14, and inductive flow member 16, respectively. Similarly, in FIG.2B capacitor 76 is coupled to line 74 via a series resistor 92.

In analyzing these circuits comparatively, it is convenient to assume avalue for real load impedance 94 that is matched to the characteristicimpedance of the delivery line. Using such a circuit simplificationresults in the respective filter circuits being terminated by aresistive load equal in value to the characteristic impedance of thedelivery line. The characteristic impedance of the delivery line isdetermined by

    Z=Sqrt[Bρ/A.sup.2 ]

where Z is the characteristic impedance (which is a real (i.e.,resistive) impedance), B is fluid bulk modulus, ρ is fluid density and Ais the inside cross-sectional area of the delivery line. In analyzingequivalent circuit 64 it is further convenient to break up the analysisas follows:

    Z.sub.1 =(Z(R.sub.3 -j/ωC.sub.2))/(Z+R.sub.3 -j/ωC.sub.2),

    Z.sub.2 =Z.sub.1 +R.sub.2 +jωL,

    Z.sub.3 =(Z.sub.2 (R.sub.1 -j/ωC.sub.1))/(Z.sub.2 +R.sub.1 -j/ωC.sub.1),

    P.sub.out /P.sub.in =Z.sub.1 /Z.sub.3,

    Z.sub.4 =(Z(R.sub.4 -j/ωC.sub.3))/(Z+R.sub.4 -j/ωC.sub.3), and

    R=Abs[(P.sub.out /P.sub.in) (Z.sub.3 /Z.sub.4)]

where (R₃ -j/ωC₂) is the series impedance of second capacitor 82 andresistor 86, Z₁ is the impedance of the parallel combination of Z and(R₃ -j/ωC₂), (R₂ +jωL) is the series impedance of the inductor 80 andthe resistor 88, Z₂ is the impedance looking toward the inductor 80, (R₁-j/ωC₁) is the series impedance of the first capacitor 78 and theresistor 84, Z₃ is the impedance of the parallel combination of Z₂ and(R₁ -j/ωC₁), P_(out) /P_(in) is the ratio of alternating pressureapplied to the delivery line divided by that present applied to thefirst capacitor 78 (i.e., that present in the first volumetricallycompliant hose section 12), (R₄ -j/ωC₃)is the series impedance of thecapacitor 76 and resistor 92, Z₄ is the impedance of the parallelcombination of Z and (R₄ -j/ωC₃), and R is the comparative ratio ofsound pressures applied to the delivery line by equivalent circuits 64and 66.

As is common in acoustic sound analysis, results of the foregoing arebest expressed in relative decibels according to the relation 20 Log[R].Results shown in FIGS. 3A and 3B for frequency ranges of 0 to 3,000 Hzand 300 Hz, respectively, were obtained for apparatus comprising thefollowing parameters: C₁, C₂ and C₃ relate to volumetrically compliantmembers formed from 4, 4 and 8 inch lengths, respectively, of highexpansion hose such as the afore mentioned Dayco No. DF 3289 (i.e.,which has an inside diameter of 0.375 in. and expands volumetrically by50% at 1,300 psi), L relates to an inductive flow member having a 0.1in. diameter 4 in. in length, and the delivery line is formed of φ10 mmsteel tubing which has an internal diameter of 0.277 in. Using thesefactors, values for each are calculated as follows:

    C.sub.1 =C.sub.2 =(0.5/(1.25 1300)) (4π0.1875.sup.2)=0.000136 in..sup.5 /lb.,

    C.sub.3 =2C.sub.1 =0.000272 in..sup.5 /lb.,

    L=(4ρ)/(π0.05.sup.2)=0.0397 lb.sec..sup.2 /in..sup.5, and

    Z=Sqrt[(Bρ)/(π0.1385.sup.2).sup.2 ]=46 lb.sec./in..sup.5,

where fluid density ρ=0.000078 lb.sec.² /in.⁴ and fluid bulk modulusB=100,000 lb./in². In addition, R₁, R₃, and R₄ are valued such thatradian corner frequencies obtained with reference to C₁, C₂, and C₃,respectively, are each 10,000 rad./sec. which results in R₁ =R₃ =0.74lb.sec./in.⁵, and R₄ =1.47 lb.sec./in.⁵. R₂ is equal to the rate of thechange of pressure drop across the inductive flow member 16 with respectto a change in flow rate therethrough or the differential dΔP_(L)/dQ_(L) across the inductive flow member 16 represented by the seriesimpedance (R₂ +jωL). Normally chosen values of fluid flow rate andgeometries for the inductive flow member 16 result in a square lawrelationship between pressure drop and flow rate therethrough whereby R₂(i.e., which is equal to dΔP_(L) /dQ_(L)) is a nominally linear functionof flow rate which can be evaluated according to coefficients andequations for turbulent flow as found, for instance, in a sectionentitled Mechanics of Fluids found in Marks' Standard Handbook forMechanical Engineers published by McGraw-Hill. In this case values havebeen chosen for which R₂ =4 lb.sec./in.⁵. Finally, delivery linecharacteristic impedance Z =46 lb.sec./in.⁵.

Shown in FIGS. 3A and 3B are curves 96a and 96b which result from thesevalues were used in evaluating the above equations. Curves 96a and 96bdepict dramatic reduction in transmitted fluid borne noise beginning atabout 133 Hz. The increased values below that frequency are due to adamped flow resonance between C₁ and C₂ via L. However, most vehicularpower steering systems exhibit their first troublesome noise frequenciesabove 200 Hz whereat curves 96a and 96b depict effective fluid bornenoise reduction.

(Although it is possible to further reduce the frequency at which unitycomparative results occur by increasing either of the capacitive orinductive section lengths, such modifications may be detrimental tooverall steering performance. For instance, excessive cumulative bypasscapacitance values may degrade system response. Further, excessivereduction of the resonant frequency denoted by curve peak 98 couldresult in a form of steering shudder wherein engine driven powersteering pump flow ripple is amplified. This typically would likelyoccur at frequencies linked to engine idle cylinder firing rates in theorder of 35 Hz commonly encountered during parking maneuvers.)

As mentioned above, curves 96a and 96b depict significant reduction influid borne noise level over frequencies of interest obtained as aresult of utilizing the improved isolating hose assembly 10 of thepresent invention. However, significant acoustic noise may still bepresent. This is because surface 100 of the first volumetricallycompliant member 12 can be driven by higher frequency harmonics, shouldthey be present, at significant vibration levels. Such high frequencysurface vibration is often sufficient for coupling sound energy into thesurrounding atmosphere in a manner similar to the action of aloudspeaker.

Therefore, in a first alternative preferred embodiment, firstvolumetrically compliant member 12 is surrounded by a non-contactinghousing whose characteristic acoustic impedance differs substantiallyfrom that of the atmosphere surrounding the first volumetricallycompliant member 12. In this way any acoustic noise generated isreflected inward and thus prevented from entering the surroundingatmosphere. As shown in FIG. 1, such a non-contacting housing is formedin the manner of a heavy-walled elastomeric boot 102 surrounding thefirst volumetrically compliant member 12. Elastomeric boot 102 comprisesend sections 104a and 104b adapted for attachment to first collarfitting 38 and collar 48, respectively, as by crimping collars 106a and106b, respectively.

Transmission of acoustic sound through elastomeric boot 102 can beanalyzed according to formula (6.41) found in a book entitledFUNDAMENTALS OF ACOUSTICS by Kinsler and Frey and published by Wiley forsound transmission through three media. Because the media on either sideof wall 108 of elastomeric boot 102 is identical (i.e., air) and becauseof the large acoustic mismatch between air and the elastomeric materialcomprised in wall 108, formula (6.41) can be reduced to the following:

    α.sub.t =4(ρ.sub.1 B.sub.1 /ρ.sub.2 B.sub.2)/Sin.sup.2 [ωSqrt[ρ.sub.2 /B.sub.2 ]]1

where α_(t) is the sound transmission coefficient from annular space 110to exterior space 112, ρ₁ B₁ is the product of air density and air bulkmodulus, ω is radian frequency, ρ₂ is elastomer density, B₂ is elastomerbulk modulus and 1 is elastomer thickness. When evaluated with thevalues ρ₁ B₁ =0.00158 lb.² sec.² /in.⁶, ρ₂ =0.000103 lb.sec.² /in.⁴, B₂=348,000 lb./in.² and 1=0.25 in., sound transmission coefficient α_(t)=15,100/ω². Conversion from radian frequency ω to frequency f results inα_(t) =382/f². At the above noted first troublesome noise frequenciesabove 200 Hz, α_(t) is less than 0.01 whereby acoustic sound is reducedby a minimum value of -20 db with values decreasing further dependingupon the square of frequency.

Combination of the above described isolating hose assembly 10 andheavy-walled elastomeric boot 102 results in a significantly improvedapparatus for reduction of fluid borne noise within and acoustic noiseemanating from hydraulic systems. Further reduction of fluid borne noisewithin and acoustic noise emanating from such hydraulic systems (albeitobtained with diminishing returns) could be obtained by extendedcombinations of additional inductive flow members and volumetricallycompliant members. Thus, one skilled in the art will readily recognizefrom such discussion, and from the accompanying drawings and claims,that various changes, modifications and variations can be made thereinwithout departing from the spirit and scope of the invention as definedin the following claims.

I claim:
 1. An apparatus for reduction of fluid borne noise in ahydraulic system, said apparatus comprising:a first and a second conduitmember, each of said first and second conduit members having a walldefining a passage having a predetermined diameter, said wall formed ofa compliant material permitting volumetrical expansion of said passagein response to an increase in pressure; an elongated inductive flowmember having a bore and a pair of ends, one of said pair of ends beingconnected to one end of said first conduit member and an other end ofsaid flow member being connected to one end of said second conduitmember, said bore having a diameter smaller than said predetermineddiameter of said passage to restrict flow of said fluid from said firstconduit to said second conduit; an input member connected to an otherend of said first conduit member to permit introduction of fluid intosaid passage; and an output member connected to an other end of saidsecond conduit to permit delivery of fluid to said hydraulic system. 2.The improved apparatus for reduction of fluid borne noise of claim 1,wherein each of said first and second conduit members comprise avolumetrically compliant hose section.
 3. The improved apparatus forreduction of fluid borne noise of claim 1, wherein said inductive flowmember comprises a tube having a bore having art end forming a flowbarrier between said first and second conduit members.
 4. The improvedapparatus for reduction of fluid borne noise of claim 1, additionallycomprising sound barrier means surrounding and spaced apart from saidfirst conduit member for preventing acoustic sound sourced at said wallthereof from entering the surrounding atmosphere.
 5. The improvedapparatus for reduction of fluid borne noise of claim 4, wherein saidsound barrier means comprises an elastomeric boot spaced apart from saidfirst conduit.
 6. A method for reducing fluid borne noise in hydraulicsystems, said method comprising the steps of:passing a flow of fluidthrough a first volumetrically compliant member to bypass alternatingfluid flow components of a hydraulic signal; passing fluid from saidfirst compliant member through a flow barrier with an inductive flowmember to impede further transmission of remaining alternating fluidflow components beyond said first volumetrically compliant member; andpassing said fluid flow from said inductive flow member into a secondvolumetrically compliant member to remove any remaining alternatingfluid flow components.